Materials used in the production of ICs (integrated circuits), such as silicon wafers and other semiconductor wafers, are handled and processed in an ultraclean environment, since the wafers are sensitive to both particulate and impurity contamination. Airborne particulates of only one micrometer in size can cause defects on the wafer surface. In a "clean room," filtered air is circulated in a vertical laminar flow pattern to eliminate particulates. Operators wear lint-free clothes, booties, gloves, and hoods to minimize particulates carried into the clean room. All chemicals, solvents, and metals used in the IC production process must be electronic grade, i.e., the highest purity grade, so as to minimize impurity contamination effects.
The impurity contamination occurs by way of the adsorption process, wherein atoms or molecules of the impurity in the solid, liquid or gaseous state are attracted by, and move onto, the surface of the substrate material. The extent of adsorption depends upon the temperature, pressure, concentration, and reactivity of the adsorbate. Substrates with high surface free energy will adsorb more readily. Adsorption saturates at about one layer of adsorbed molecules. The adsorbed monolayer exhibits lower free energy than a clean surface, and so the tendency to adsorb further is decreased as the monolayer is formed. Semiconductor wafers are hydrophilic, and adsorb polar molecules such as water very strongly. Thus, by allowing water to adsorb onto the substrate, further adsorption of impurities, especially highly reactive hydrocarbons, is greatly reduced.
During the manufacturing process, wafers are often transferred from one clean room to another clean room, and are inadvertently contaminated during the move. This necessitates recleaning upon arrival at the new station, a painstaking and time consuming process.
Current methods used to transport wafers without contamination have proven inadequate. One such method is to wrap the wafers in vacuum shrink plastic film while in the clean room. This is a clumsy procedure which does not ensure cleanliness. Another method is to place the wafers within a small vacuum chamber for transport. The vacuum may exacerbate the problem since any residual gas, carbon containing contaminants, or any leakage, even in an ultra high vacuum, can easily adsorb on the unprotected wafer surface. The leaks may be real, where the vacuum draws gases in from outside the vacuum chamber, or virtual, where contaminants outgas off material within the vacuum chamber. This is most serious prior to a deposition process, where less than a monolayer of adsorbate can grossly degrade the interfacial state, thereby impairing the performance of the interface and ultimately the semiconductor device. Furthermore, the vacuum chamber is a heavy system, and requires a large pump and power supply. This limits the portability of the vacuum chamber transport system. Accordingly, there is a need to provide a lightweight portable system for transporting materials that will eliminate particulate and chemical contamination.